how to find the APIs of the ip in nios?

Almost every IP core comes with some documentation, which can be more or less complete.

If it's an IP from Altera or if the IP has been designed as a Qsys component, usually you have the HAL driver or, at least a register map to access it through IOWR/IORD.

If you need to integrate a generic IP (i.e. from opencores.org) you may need to add a wrapper to interface it to Avalon bus, create a new custom device in Qsys and write the APIs yourself.